1. Field of the Invention
The invention relates to an apparatus for transmitting radio-frequency signals using a signal generation unit, a signal line, a radiating element and a waveguide which is terminated in an end region by a back wall, where the signal generation unit generates the radio-frequency signals, where the signal line routes the radio-frequency signals to the radiating element of the waveguide, and where the radiating element projects into the waveguide.
2. Discussion of Related Art
An apparatus of the type described above is used, for example, in instruments which determine the level of a filled product in a container using the delay time of radio-frequency measurement signals. Pulse timing methods utilize the physical conformity to law according to which the distance covered is equal to the product of delay time and propagation speed. In the case of level measurement, the distance covered is equivalent to twice the distance between antenna and surface of the filled product. The useful echo signal, that is to say the signal reflected at the surface of the filled product, and the delay time thereof are determined using the xe2x80x98echo functionxe2x80x99 or using the digitized envelope, the envelope representing the amplitudes of the echo signals as a function of the distance xe2x80x98antennaxe2x80x94surface of the filled productxe2x80x99. The level itself is then found from the difference between the known distance from the antenna to the bottom of the container and the distance from the surface of the filled product to the antenna which is determined by the measurement.
Customary methods for determining distance using the delay time of electromagnetic signals are the pulse radar method and he frequency modulation continuous wave method (FMCW method). In the case of the pulse radar method, short microwave pulses are transmitted cyclically. In the case of the FMCW method, a continuous microwave is Transmitted which is cyclically subjected to linear frequency modulation, for example on the basis of a sawtooth function. The frequency difference between the frequency of the received echo signal and the frequency of the transmitted signal at the instant of reception is dependent on the delay time of the echo signal. The frequency difference between transmitted signal and received signal, which difference can be obtained by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance between the reflector, e.g. the surface of the filled product, and the antenna. In addition, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes, which means that the Fourier spectrum represents the echo function.
The propagation of the radio-frequency measurement signals in the signal line and in the waveguide conforms to the physical laws of the propagation of electromagnetic waves. Normally, the signal line is a coaxial line. An input element is used to route the radio-frequency measurement signals from the inner conductor of the coaxial cable to the radiating element of the waveguide. The waveguide is either in the form of a square waveguide or is in the form of a round waveguide, with antennas having a circular cross section preferably being used in the field of level measurement, since they are better suited to being fitted into the nozzle of a container (tank, silo etc.), for example, than waveguides having a square cross section.
In a coaxial line, the transverse electromagnetic mode (TEM mode) ideally propagates without dispersion. This TEM mode is therefore particularly well suited to transporting wave packets or electromagnetic waves having a certain bandwidth. Wave packets which propagate in the TEM mode therefore encounter no spreading; similarly, microwaves frequency modulated on a linear basis largely prevent any discrepancy in linearity.
For the directional transmission of electromagnetic waves using an antenna, a mode is preferably used whose radiation characteristic has a pronounced forward lobe. This is a property of the transverse electric fundamental mode capable of propagation in round waveguides, the TE11 mode. In a square waveguide, the corresponding fundamental mode is the TE10 mode. Depending on the dimensions of the antenna in waveguide form, there is a respective defined frequency range in which exclusively this fundamental mode is capable of propagation. Above this frequency range, higher modes less well suited to directional transmission of microwaves propagate as well, for example the TM01 mode in the case of the round waveguide and the TE20 mode in the case of the square waveguide. While the range of unambiguity, that is to say the range in which only the fundamental mode is capable of propagation, is relatively large for a square waveguide, the range of unambiguity in the case of a round waveguide has relatively narrow proportions. The likelihood of undesirable higher modes also being prompted in addition to the fundamental mode when broadband signals are input is therefore much greater in the case of a round waveguide than in the case of a square waveguide. One undesirable consequence of different modes developing is xe2x80x98ringingxe2x80x99. Ringing is caused by virtue of the fact that the individual modes capable of propagation in a waveguide have different propagation speeds. This is manifested in that the transmitted pulse does not disappear abruptly, but rather loses amplitude slowly. This ringing edge can cover the echo signal in the measurement range or can have the echo signal superimposed on it such that relatively large errors may arise when determining the measured value.
As an aside, examples of level-measuring instruments which have been disclosed to date are described in EP 0 821 431 A2 and in DE-GM 93 12 251.9. While EP 0 821 431 A2 describes an embodiment in which the radiating element, the xe2x80x98transmission wirexe2x80x99, is routed through the back wall into the interior of the waveguide, in DE-GM 93 12 251.9, the radio-frequency measurement signals are input onto the waveguide through the side wall.
The invention is based on the object of proposing an apparatus for transmitting radio-frequency measurement signals which is distinguished by an optimized radiation characteristic.
The object is achieved by virtue of the radiating element being arranged at an angle to the back wall of the waveguide or to a plane of the waveguide which is parallel to the back wall.
Known solutions always assumed that the radiating element, that is to say the exciter pin, needs to be arranged parallel to the back wall of the waveguide for optimum E field input.
Surprisingly, however, it has been found that much better results can be achieved when the exciter pin does not run parallel to the back wall, but rather at a certain angle to the back wall or to a plane which is parallel to the back wall. This angle depends on the rest of the input geometry and cannot be defined generally. It has been foundxe2x80x94as already mentionedxe2x80x94that the exciter pin""s being at an angle produces the stimulus in a very much more single-mode fashion, i.e. in essence only the desired mode, that is to say the fundamental mode, is prompted. This single-mode input can also be achieved even when measurement signals with a very broad bandwidth are input onto the waveguide. In addition, the apparatus according to the invention achieves very good matching between the signal line and the input element. As a result of the two effects, the ringing already described previously is drastically reduced, particularly when broadband measurement signals are input. In addition, suppression of the undesired higher modes achieves the desired radiation response with a pronounced directional characteristic in the direction of radiation.
In accordance with a first refinement of the apparatus according to the invention, the radiating element is routed through the back wall of the waveguide. In an alternative embodiment of the apparatus according to the invention, the radiating element is routed through the side wall of the waveguide.
In accordance with one advantageous development of the apparatus according to the invention, the size of the angle between the radiating element and the back wall of the waveguide or a plane parallel to the back wall of the waveguide is greater than 4xc2x0.
The radiating element may be a transmission wire, for example. In this case, a transmission head is preferably arranged in the region of the free end of the transmission wire. Alternatively, the radiating element used may be a conductor structure which is arranged on a printed circuit board, the radiating structure being arranged on the printed circuit board at an angle to the back wall of the waveguide or to a plane which is parallel o the back wall of the waveguide.
In accordance with one advantageous refinement of the apparatus according to the invention, the waveguide is routed to a horn, rod or parabolic antenna. This also allows the radiation characteristic of the apparatus to be optimized.
To protect the radiating element from deposits, one advantageous embodiment of the apparatus according to the invention provides for a dielectric material to fill up at least the interior of the waveguide in the region of the transmission wire. In particular, it is proposed in this context that the dielectric material contain a recess into which the transmission wire projects. The dielectric material is polytetra-fluoroethylene (PTFE) or aluminum trioxide (Al2O3), for example.
As already mentioned previously, the apparatus according to the invention is preferably part of a level-measuring instrument. However, use of the apparatus according to the invention is in no way limited to this use. In principle, the apparatus can be used in any apparatuses which use radio-frequency measurement signals.